1 Corinthians 13 is one of the best-known passages in all of Paul - partly, I suspect, because many couples still choose to have it read in public at their wedding, though if they reflected on it line by line they might find it quite a daunting challenge:
Love is great-hearted; love is kind, knows no jealousy, makes no fuss, is not puffed up, has no shameless ways, doesn’t force its rightful claim; doesn’t rage, or bear a grudge, doesn’t cheer at others' harm, rejoices, rather, in the truth. Love bears all things, believes all things; love hopes all things, endures all things. Love never fails...
The love of which Paul speaks is clearly a virtue.
It is not a "rule" of the sort that is so out of fashion nowadays, imposed in an arbitrary fashion and to be obeyed out of a sense of duty. (We shall discuss the more serious question of proper rules and their relation to virtue later on.)
It is not a "principle", a generalised rule which a person either obeys or disobeys.
It is not a "prudential maxim based on calculated effects"; though it has to be said that if even a few more people lived in the way Paul describes, a lot more people would be a lot more happy.
Nor, especially, is it the result of people "doing what comes naturally". At every single point in Paul's catalogue of what love does, and what love doesn't do, we want to say, "Yes, I see what you mean. However, left to my own inclinations, I would be small-minded, unkind, jealous, fussy, puffed up, shameless, and so on. In particular, left to myself, there are some things I wouldn't bear, many things I wouldn't believe, several things I wouldn't be able to hope for, and a whole multitude that I wouldn't endure. Left to myself, doing what comes naturally, I would fail." But the point of love is that it doesn't [fail].
That is why love is a virtue. It is a language to be learned, a musical instrument to be practiced, a mountain to be climbed via some steep and tricky cliff paths but with the most amazing view from the top. It is one of the things that will last; one of the traits of character which provides a genuine anticipation of that complete humanness we are promised at the end. And it is one of the things, therefore, which can be anticipated in the present on the basis of the future goal, the telos, which is already given in Jesus Christ. It is part of the future which can be drawn down into the present. - N.T. Wright After You Believe